The use of composite structures in aircraft is beneficial due to their high strength and low weight. Often a metal part can be replaced by a composite structure at one-tenth the weight. The composite structure often consists of a low density core covered by a resin impregnated fabric, or prepreg, in a "layup." The core is typically a block structure with angled ends, or "ramps", to facilitate application of the o prepreq. The core may be comprised of a plurality of hexagonal or over-expanded cells adjacent and attached to each other. The layup is then cured in an autoclave at temperatures of 250.degree. to 400.degree. Fahrenheit and pressures of 30-100 pounds per square inch. At these temperatures and pressures, the composite structure will cure. While curing, the resin in the prepreg will soften, flow, and permeate the upper portions of the core.
A problem with this method of making composite structures develops due to the pressure differential between the autoclave and the composite structure core. The pressure exerted may cause the low density core to collapse or shift, deforming the structure. A number of patents disclose methods which attempt to overcome this problem through the use of internally applied pressure. These include U.S. Pat. No. 3,138,506 to Ross, U.S. Pat. No. 3,962,506 to Dunahoo, and U.S. Pat. No. 4,169,749 to Clark. For example, Ross discloses a method of making an article of stiffened fibrous material which includes the use of fibrous material formed under pressure with the article being inflated during the process. A tubular valve or inflation nipple may be built into or attached to the closed body to permit inflation. Additionally, Dunahoo discloses a multi-chamber cellular structure and method of manufacture which includes a plurality of shells. The shells are composed of a plastic having embedded therein fibrous reinforcing material. During the manufacturing process, pressure is introduced inside the structure so as to expand the structure into a mandrel.
A second problem develops during the processing of composite structures in an autoclave due to the emission of volatiles from the materials used in the core. During the curing process, these gases can bubble out of the materials, causing voids or unsmooth surfaces. Alternatively, the core may "seal off" allowing internal gas pressure to increase to unsafe levels. In either case, the composite structure is damaged. Several methods have been developed to facilitate the removal of volatiles from the layup during the curing process. These include U.S. Pat. No. 4,816,106 to Turis et al., U.S. Pat. No. 4,132,755 to Johnson, and U.S. Pat. No. 3,937,778 to Tanaka. For example, Turis et al. discloses a method of controlled curing of composites which includes means for removing volatiles from the layup during the curing process. The layup is deposited over a release material adjacent to the mold. Volatiles removal is then accomplished by depositing a breather strip about the periphery of the layup. The layup and the breather material are enclosed within a vacuum bag which is sealed to the mold by means of sealing strips. A connecting tube is then provided to place the breather material in communication with the vacuum source. The layup is then heated to a temperature which will permit the escape of volatiles.
A need exists for a method or process of pressure balanced processing of co-cured composite sandwich structures. The present invention provides such a method which addresses the problems of core collapse and the emission of volatile gases, and is suitable for use in an autoclave environment.